Quality control of sheet materials like paper can be crucial for industrial processes such as paper processing (e.g. in paper production), for reasons that range from economical optimization to physical performance. There are two types of quality control: off-line and in-line. Off-line quality control takes place outside of the paper processing, e.g. as post processing verification, and is often carried out in a laboratory. In contrast, in-line quality control takes place in real time as part of the paper processing. In-line quality control can, for example, be performed at the dry end of the process and may serve as feedback for the wet end process. Parameters determined for in-line quality control of paper can include caliper (thickness), basis weight, moisture, ash, brightness, gloss and color of the paper. Other parameters like coat weight and fiber orientation and anisotropy may be determined as well.
Current in-line quality control systems typically consist of heavy weight frames on which a sensor head box is mounted containing many sensor elements, each of which is used to characterize an individual parameter of the running paper sheet. The sensor head scans perpendicular to the paper feed direction to obtain a representative measure of the quality of the processed material. Some of the sensors of a typical sensor head may require beta radiation which needs particular shielding and care. Some other sensors require physical contact with the paper sheet material and may therefore interfere with the paper handling, e.g. by increasing the risk of damaging the paper.
Recently, a contactless measurement technique using THz radiation has been proposed. A system for in-line measurements of sheet caliper based on THz radiation has been described in US 2009/0128799 A1.
Nevertheless, there is still a need for a system that extracts the relevant quality parameters from a THz measurement signal in a reliable manner for obtaining robust and meaningful quality parameters.